Lighting systems utilizing light emitting diodes (LEDs) are becoming increasingly popular and include systems employing constant current and constant voltage power supply architectures. Constant current systems may employ a power supply having a control system to output a desired current, within certain design parameters, irrespective of the number of LEDs attached to the power supply. In contrast, constant voltage power supply architectures require the number of LEDs and forward voltages across the LEDs to be accounted for in order to achieve adequate performance in many applications.
Constant voltage power supply architectures may employ power supplies having an adjustable output voltage that may be adjusted to suit a particular application. For example, a 24V power supply may be adjusted to have an output voltage of approximately 22V to provide a source of power to a group of seven LEDs connected in series between the power supply output and a reference ground. However, constant voltage power supplies having an adjustable output are more expensive and difficult to source compared to constant voltage power supplies having a fixed output voltage.
A constant voltage power supply having a fixed output voltage may be used with a voltage converter to achieve the desired output voltage for a particular application. The voltage converter may be connected to the output of the power supply to increase or decrease the voltage drop between the voltage converter output and a return path or low voltage rail. However, complications may arise as a result of LEDs having different forward voltages as a result of manufacturing tolerances or otherwise having different characteristics. Similarly, the forward voltage and current voltage characteristics may change as a result of changes in the temperature of the LEDs, which may affect the current flowing through the LEDs and therefore the intensity of light radiated from the LEDs. Discrepancies in the forward voltages of the LEDs may be problematic when various LED channels are connected in parallel.
Certain other known LED architectures that utilize pulse-width modulation (PWM) may result in relatively large current fluctuations that may increase as the total current through the LEDs increases. One example of an existing solution that mitigates against excessive current fluctuations and permits the intensity of light output from a plurality of LEDs to be varied is U.S. Pat. No. 7,759,881 issued Jul. 20, 2010 to Melanson. Melanson discloses a system employing a different dimming scheme based on the intensity of light output. For relatively high intensities (i.e. limited dimming), a feedback control system is used to adjust the current output from a voltage converter. At lower intensities, a low frequency PWM signal is also modulated with the control signal to further limit the average current flowing through the LEDs. However, the control architecture in Melanson is limited and does not permit aspects of the light output, such as the color or color temperature, to be varied.
Other architectures having multiple LED strings or blocks in parallel are configured to effectively control each block independently. For example, U.S. Published Patent Application No. 2009/0134817 published May 28, 2009 (Jurngwirth) discloses an architecture having a number of LED blocks that each have a separate voltage converter and control system between the power supply and ground so that each block is controlled independently. This architecture may be relatively complex and result in large current fluctuations in the current drawn from the power supply.
Certain other existing control systems for lighting apparatus utilizing a voltage converter may also have limited resolution to control the output voltage from the voltage converter. Additionally, the relationship between various control signals used in the lighting apparatus may contribute to a visible flicker of the light output from the lighting apparatus under certain operation conditions.
It would be desirable to develop a modular LED architecture that mitigates against known problems in the prior art.